The present invention relates to a method of producing a method of producing a printed wiring board for a novel ball grid array type semiconductor plastic package in which at least one semiconductor chip is mounted in a printed wiring board, and also relates to a printed wiring board. The printed wiring board produced according to the present invention is suitable for use in a relatively high-power, multi-terminal and high-density semiconductor plastic package such as a microprocessor, a micro-controller, an application specific IC (ASIC), a graphic terminal, or the like. The semiconductor plastic package is mounted on a mother board printed wiring board with solder balls, and the mother board is used for an electronic machine or equipment.
As a semiconductor plastic package, a plastic ball grid array (P-BGA), a plastic land grid array (P-LGA), etc., are known. Such a semiconductor plastic package has a structure in which a semiconductor chip is fixed on an upper surface of a plastic printed wiring board, the chip is connected to a conductor circuit formed on the upper surface of the printed wiring board by wire bonding, conductor pads are formed on the lower surface of the printed wiring board, the conductor pads are formed of solder balls for connecting the semiconductor plastic package to a mother board printed wiring board, the conductor circuit on the front and reverse surfaces are connected through plated through hole(s), and the semiconductor chip is encapsulated with a resin. For diffusing heat generated in the semiconductor into the mother board printed wiring board, the above structure is also provided with plated heat-diffusing through hole(s) which lead or leads from a metal foil which is on the upper surface and on which the semiconductor chip is mounted to the reverse surface.
Moisture is absorbed into a silver-powder-containing resin adhesive used for fixing the semiconductor chip, and the moisture may cause a inter-layer swelling under heat through the through hole(s) when the solder balls are heated for mounting the plastic package on the mother board or removing it from the mother board. The above swelling is called a popcorn phenomenon. When the popcorn phenomenon takes place, the package is no longer usable in many cases, and it is required to prevent the occurrence of the pop-corn phenomenon drastically.
Further, higher function and higher density of a semiconductor mean an increase in heat generated, and the forming of only through holes immediately below a semiconductor chip comes to be insufficient for heat diffusion.
It is an object of the present invention to provide a method of producing a printed wiring board for a ball grid array type semiconductor plastic package which has excellent heat diffusibility and causes no popcorn phenomenon, and a printed wiring board.
According to the present invention 1, there is provided a method of producing a printed wiring board for use in a ball grid array type semiconductor plastic package, the printed wiring board having a structure in which a metal plate is placed in a region located as part of the thickness direction of a printed wiring board, the metal plate has a flat region in one surface, a plurality of metal protrusions having the form of frustum of a cone each are formed in the other surface of the metal plate so as to be connected to a copper foil forming a reverse surface, the metal plate has a size nearly equivalent to the size of the printed wiring board, a semiconductor is fixed on the flat region located at a level lower than the level of a circuit and a circumferential bonding pad on the front surface of the printed wiring board, the metal plate and a signal propagation circuit conductor on the front surface of the printed wiring board are insulated from each other with a thermosetting resin composition, the said circuit conductor and the semiconductor chip are connected by wire bonding, and at least the signal propagation circuit conductor on the front surface of the printed wiring board and a circuit conductor formed on an opposite surface of the printed wiring board or circuit conductor pads formed on the opposite surface of the printed wiring board for connecting the package to another circuit with solder balls are connected with through hole conductor,
the method comprising the steps of
a. etching the metal plate of a metal-clad laminate board formed of the metal plate, a glass fabric base material/thermosetting resin composition prepreg sheet and a copper foil which are laminated in this order, to form an elevated portion in that region of the metal plate which is opposed to the flat region where the semiconductor is to be mounted,
b. forming protrusions having the form of frustum of a cone each in the elevated portion and forming a circuit around the elevated portion,
c. placing a glass fabric base prepreg sheet on a region including said circuit but excluding an area of the protrusions, the prepreg sheet having a hole which is made so as to correspond to a region of the metal protrusions and has a size little larger than the region of the metal protrusions, placing a copper foil on the prepreg sheet and the area of the hole, and forming the resultant set into a laminate under heat and pressure to form a double-side copper-clad laminate,
d. making penetration holes for through holes in a region other than the area of the metal protrusions having the form of frustum of a cone each, plating the penetration holes with copper, and removing copper foil, glass fabric base and resin layer on at least the flat region where the semiconductor chip is to be mounted, to expose a metal surface,
e. forming a bonding pad and a circuit for signal propagation on a region around the flat region where the semiconductor chip is to be mounted, forming, on the opposite surface, solder ball pads for connecting the metal foil with which the metal protrusions having the form of frustum of a cone each are in contact, to solder balls for heat diffusion and forming a circuit on the opposite surface, and
f. plating at least a semiconductor-chip-mounting portion, a wire bonding pad portion and a solder ball pad portion with a noble metal.
According to the present invention 2, there is provided a method of producing a printed wiring board according to the above, wherein a circuit including a bonding pad which is to be connected to the semiconductor chip by wire bonding is formed on two levels, the method comprising the steps of
a. forming a circuit in a region of one surface of a glass fabric base material/double side metal-clad laminate which region is other than a semiconductor-chip-mounting portion but is on a front surface of the laminate, and forming a plurality of metal protrusions having the form of frustum of a cone each and a circuit on the other surface of the laminate which surface is the reverse surface,
b. placing a glass fabric base material/thermosetting resin composition prepreg sheet on the front surface, placing a metal foil thereon, placing a glass fabric base prepreg sheet on the reverse surface, the glass fabric base prepreg sheet having a hole which is made so as to correspond to a region of the metal protrusions and has a size a little larger than the region of the metal protrusions, placing a metal foil thereon, and forming the resultant set into a laminate under heat and pressure to obtain a double-side metal-clad multi-layered board,
c. making penetration holes for through holes in a portion which is present neither in the semiconductor-chip-mounting portion nor in the region of the productions having the form of frustum of corn each, plating the penetration holes with a metal, removing at least metal foil, glass fabric base and resin layer on the semiconductor-chip-mounting portion on the front surface, to expose a bonding pad of a second level, and then exposing a metal surface which is to constitute the semiconductor-chip-mounting portion, and
d. forming at least a bonding pad of a first level and a circuit on the front side, forming, on the reverse surface, solder ball pads for connecting heat-diffusing solder balls to a surface metal foil to which the metal protrusions having the form of frustum of a cone each are in contact, forming a circuit on the reverse surface, and plating at least the semiconductor-chip-mounting portion, a wire bonding pad portion and a solder ball pad portion with a noble metal.
According to the present invention 2, there is also provided a method of producing a printed wiring board according to the above, wherein the metal foil forming a surface layer of the multi-layered board is removed, the thermosetting resin composition and the glass fabric base layer are cut off by a sand blasting method until the bonding pad of the second level is reached, the bonding pad of the second level is coated, and then, the resin layer and the glass fabric base layer are cut off by a sand blasting method until the metal plate is reached, whereby the bonding pad portion of the second level and the semiconductor-chip-mounting portion are exposed.
According to the present invention 3, further, there is provided a metal-plate-inserted printed wiring board having wire bonding pads formed at two levels, for use in a ball grid array type semiconductor plastic package, the printed wiring board having a structure in which the metal plate is placed in a region located as part of the thickness direction of a printed wiring board, the metal plate has a flat region in one surface, a plurality of metal protrusions having the form of frustum of a cone each are formed in the other surface of the metal plate so as to be connected to a copper foil forming a reverse surface, the metal plate has a size nearly equivalent to the size of the printed wiring board, a semiconductor is fixed on the flat region located at a level lower than the level of a signal propagation circuit and circumferential bonding pads of two levels on one surface of the printed wiring board, the metal plate and a signal propagation circuit conductor on the front surface of the printed wiring board are insulated from each other with a thermosetting resin composition, the signal propagation circuit conductor and the semiconductor chip are connected through the bonding pads of two levels by wire bonding, and at least the signal propagation circuit conductor on the front surface of the printed wiring board and a circuit conductor formed on an opposite surface of the printed wiring board or circuit conductor pads formed on the opposite surface of the printed wiring board for connecting the package to an outside with solder balls are connected with a through hole conductor insulated from the metal plate with a resin composition,
the printed wiring board being produced by providing a metal plate, one surface of the metal plate having a flat region where the semiconductor is to be mounted, the other surface of the metal plate having a plurality of metal protrusions having the form of frustum of a cone each, the metal plate having a clearance hole or a slit in a region which is neither a semiconductor-chip-mounting portion nor a region of the metal protrusions having the form of frustum of a cone each, placing a glass fabric base material/thermosetting resin prepreg on one surface of the metal plate, placing a glass fabric base material/thermosetting resin prepreg on the other surface of the metal plate, placing a double-side copper-clad laminate having a bonding pad of the second level and an inner layer circuit on the prepreg on one surface such that the circuit faces inward, placing a glass fabric prepreg sheet on the prepreg on the other surface, the glass fabric prepreg sheet having a hole made so as to correspond to the region of the metal protrusions and so as to have an area a little larger than the region of the metal protrusions, placing a copper foil on the glass fabric prepreg, forming the resultant set into a laminate under heat and pressure to obtain a double-side copper-clad multi-layered board, making a through hole nearly in the center of the clearance hole or the slit so as not be in contact with the metal plate, plating the through hole with copper, removing copper foil, glass fabric base and resin layer on a semiconductor-chip-mounting portion and a bonding pad portion of the second level on a front surface, to expose the bonding pad of the second level, exposing part of the metal plate for the semiconductor-chip-mounting portion,, forming the bonding pad of the first level and a circuit on the front surface, forming solder ball pads for connecting the protrusions of the metal plate to heat-diffusing solder balls and a circuit in a copper foil to which the protrusions of the metal plate are in contact, and plating at least the semiconductor-chip-mounting portion side, a wire bonding pad portion and a solder ball portion with a noble metal.